Defects of vision

December 6, 2018

The disorder in which a person cannot see nearby or distant objects clearly is called defect of vision.
There are two most common defects of vision. They are:

a)Myopia

It is the defect of vision in which a person can see nearby objects clearly but cannot see distant objects.
It is also called short sightedness or near sightedness.
For such an eye, the far point is less than infinity.

Causes

Due to the shortening of the focal length of the eye lens or high converging power of the eye lens.
Due to the elongation of the eye balls.

In this case the ciliary muscles attached to the eye lens do not relax sufficiently to make the eye lens thinner to reduce the converging power.
So, the image of a distant object is formed in front of the retina due to the great converging power of the eye lens.
As a result, the person cannot see distant objects clearly.

Remedy

Myopia can be corrected by using concave lens of suitable focal length.
When a concave lens of a suitable power is placed in front of the eye having myopia, the parallel rays of light coming from the distant object are first diverged by the lens.
Due to the combined action of both lenses, the image of the distant object is formed at the retina.
As a result, the person can see the distant objects clearly.

b) Hypermetropia

It is the defect of vision in which a person can see distant objects clearly but cannot see nearby objects.
It is also called long sightedness, hyperopia or far sightedness.
For such an eye, the near point is more than 25 cm.

Causes

Due to increase in the focal length of the eye lens or low converging power of the eye lens.
Due to the contraction of the eyeball.

Remedy

It can be corrected by using a convex lens of a suitable focal length.
When a convex lens of suitable focal length is placed infront of the eye, the rays of the light coming from a nearby object are first converged by the convex lens and then by the eye lens.
Due to the combined converging action of both the lenses, the image is formed at the retina.
As a result, the person can see nearby objects clearly.

c)Astigmatism

This defect is when the light rays do not all come to a single focal point on the retina, instead some focus on the retina and some focus in front of or behind it.
This is usually caused by a non-uniform curvature of the cornea.
A typical symptom of astigmatism is if you are looking at a pattern of lines placed at various angles and the lines running in one direction appear sharp whilst those in other directions appear blurred.
Astigmatism can usually be corrected by using a special spherical cylindrical lens; this is placed in the out-of-focus axis.